Extra‐pair paternity in the socially monogamous mountain bluebird Sialia currucoides and its effect on the potential for sexual selection

Abstract

Sexual selection theory posits that ornamental traits can evolve if they provide individuals with an advantage in securing multiple mates. That male ornamentation occurs in many bird species in which males pair with a single female is therefore puzzling. It has been proposed that extra-pair mating can substantially increase the variance in reproductive success among males in monogamous species, thus increasing the potential for sexual selection. We documented the frequency of extra-pair paternity and examined its effect on variation in male reproductive success in the mountain bluebird Sialia currucoides, a socially monogamous songbird in which males possess brilliant plumage ornamentation. Extra-pair paternity was common in our Wyoming study population, with 72% of broods containing at least one extrapair offspring. The standardized variance in actual male reproductive success (i.e., the total number of within-pair and extra-pair offspring sired) was more than seven times higher than the variation in apparent success (i.e., success assuming that no extra-pair mating occurred). Success at siring within-pair and extra-pair offspring both contributed to the variation in overall male reproductive success. Within-pair success, however, did not predict a male's level of extra-pair success, suggesting that males do not sacrifice within-pair paternity to gain extra-pair paternity. Calculation of the sexual selection (Bateman) gradient showed that males sire approximately two additional offspring for each extra-pair mate that we identified. Thus, in this sexually dichromatic species, extra-pair mating increases the variance in male reproductive success and provides the potential for sexual selection to act.

Mendel’s law reveals fatal flaws in Bateman’s 1948 study of mating and fitness

“…Studies reporting results inconsistent with Bateman’s principles 17 - 20 and the many studies showing a benefit of polyandry 21 - 24 for females implicitly reject Bateman’s conclusion that selection does not favor female multiple mating. The studies that demonstrated consistency with Bateman’s principles 4 , 6 , 25 - 28 piqued our interest further, as alternative explanations for Bateman’s observations besides sexual selection have also been available for at least 25 years; yet few workers have tested their observations against these alternatives 15 , 29 . A repetition could have put our minds to rest about the validity of Bateman’s conclusions and might have provided an unbiased evaluation of mate number and reproductive success that would allow robust tests of the currently obvious alternatives 15 , 29 .…”

The influence of male age on within‐pair and extra‐pair paternity in passerines

“…In addition, the success of older males may influence the strength of selection acting upon older age classes, with a potential impact upon reproductive senescence (Graves 2007). The lack of a negative correlation between EPP and WPP effect sizes also suggests that across species the success of older males in siring extra‐pair young does not result in paternity losses with their social mates (Balenger et al. 2008).…”

Reproductive correlates of plumage coloration of female Mountain Bluebirds

“…Although socially monogamous, the rate of extra‐pair paternity is high in this species (72% of broods; Balenger et al. ), thus providing the opportunity for sexual selection. Because both sexes contribute to offspring provisioning and territory defense (Gibson and Moehrenschlage ), mutual mate choice is expected to be beneficial (Clutton‐Brock ).…”

“…To date, few investigators have applied the Good Parent and Differential Allocation hypotheses to females to test whether plumage coloration is an indicator of the extent of parental care. Among studies that have been conducted, results are mixed and have revealed positive (Linville et al 1998, Jawor et al 2004, Siefferman and Hill 2005, negative (Badyaev and Hill 2002, Johnsen et al 2005, Osmond et al 2013, and no relationships (Rohde et al 1999, Smiseth and Amundsen 2000, Balenger et al 2007) between attractiveness and parental care.…”